1. Field of the Invention
This invention relates to phenol-formaldehyde resinous compositions.
2. Description of the Prior Art
Phenol-formaldehyde condensates in the form of resoles are used extensively to impregnate paper substrates. Upon curing to a substantially insoluble and infusible stage, the resinous binder imparts structural strength and chemical resistance to the substrate for use in a variety of applications. A specific, but foremost application of the use of a phenolic resole in this manner is in the manufacture of filter media for both air and oil filtering systems in connection with the operation of on-and-off-the-road internal combustion engines.
The fabrication of filter structures of the foregoing type involve the following manufacturing operations. In the first or treating step, a continuous roll of paper is conventionally impregnated with a phenolic resole in the form of an alcohol solution of a condensate of phenol with formaldehyde. The saturated paper is heated through ovens or can driers to remove solvent. The treated paper is then corrugated for the purpose of increasing surface area. The corrugated sheet is subsequently conveyed through an oven in order to advance the cure of the resinous impregnate to a fusible intermediate or B stage, and then rolled again. B staging may be immediate to or time removed from the saturation step.
The rolls of the partially cured corrugated impregnated paper are provided in this manner to the filter manufacturer for completion of the manufacturing sequence. The latter initially involves appropriately pleating the paper and then heat-curing same to the final thermoset stage in order to achieve a sought degree of chemical, oil and moisture resistance for the filter medium. In the curing of resoles of the type referred to above in the preparation of paper filters, a copious amount of blue smoke is generated. This smoke is discharged into the atmosphere and because of its noxious nature poses an environmental problem. The current restrictions of the EPA in some states limit such discharge to those effluents which can pass the 80% light transmission test. Beyond meeting the foregoing requirement, it is desirable to reduce smoking because in so doing one is minimizing the loss of valuable phenol. Additionally, discharge effluents high in phenol derivatives content eventually form deposits on the blower fan, thereby impeding the efficiency thereof. Also, such deposits are prone to drip back into the oven and thus contaminate the paper undergoing curing.
The problem of smoking has to a large extent been obviated by substantially increasing the amount of formaldehyde employed in preparing the resole. This approach basically leads to the formation of phenol-formaldehyde polymers of higher average cross-linking density which are less apt to be emitted in the form of smoke under the conditions necessary for converting the phenolic condensate to a thermoset stage. However, the disadvantage of this approach is that the cured phenolic resin becomes too brittle for use in filter paper applications. Accordingly, the object of this invention is to provide a phenolic resole which will cure to a thermoset stage without any appreciable degree of smoking and yet the resultant cured product will retain sufficient flexibility for use in preparing paper filters.